Phagocytic cells possess multiple mechanisms for the destruction of invading organisms. One important line of attack involves the generation of activated oxygen species and derivatives thereof (such as halogens). These damage various microbial components and aid in destruction of the invading organism. The importance of oxidant killing of bacteria is emphasized by experiments of nature such as chronic granulomatous disease and myeloperoxidase deficiency. In the former, failure of initial one- electron reduction of oxygen renders phagocytes incapable of any form of oxidant killing. Consequently, affected patients are subject to frequent infection with aerobic catalase-positive bacteria. Rare patients deficient in myeloperoxidase activity may also be subject to infection but are not so severely at risk. Finally, the failure of oxidative killing may be most important in the case of an elite group of pathogens which occupy the extraordinary econiche of the normal phagosome. Organisms such as Leishmania, Legionella, and Mycobacter may survive and replicate within macrophage/monocyte phagocytic vacuoles, thereby gaining protection from recognition and destruction by host neutrophils, antibodies, and complement. Our investigations are meant to explore the hypothesis that survival of these organisms depends, in part, upon their having hypertrophied oxidant defense mechanisms. The importance of variations in pathogen oxidant defense parameters is not well defined. Employing bacteria genetically manipulated to have higher or lower activities of oxidant defenses - including superoxide dismutase, catalase, and glutathione - we shall determine the effects of deficiencies and excesses in these systems on the ability of the bacteria to survive exposure to oxidants such as 02-, hydrogen peroxide and H0C1. In concurrent studies, we shall determine the ease of phagocyte destruction of the organisms. These experiments will permit, for the first time, direct assessment of the importance of pathogen oxidant defense mechanisms. The results should not only improve our understanding of phagocyte oxidant destruction mechanisms but also provide clues as to necessary steps in the natural evolution of intraphagocytic pathogenic organisms.